Closes #254 Replace exprs(STUDYID, USUBJID) with get_admiral_option("subject_keys") in by_vars input of derive_vars_merged()

[steventing12]

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[manciniedoardo]

@steventing12 as discussed, you need to add get_admiral_option to the right bit of R/admiralophtha-package.R

[github-actions]

Package	Line Rate	Health
admiralophtha	100%	✔
Summary	100% (133 / 133)	✔

[manciniedoardo]

@steventing12 FYI the reason the tests were failing is because you hadn't added get_admiral_option() to the list of imported functions from {admiral} in admiralophtha-package.R - now I've done it and it all looks good!